Odontostilbe microcephala Eigenmann, 1907

Chuctaya, Junior, Bührnheim, Cristina M. & Malabarba, Luiz R., 2018, Two new species of Odontostilbe historically hidden under O. microcephala (Characiformes: Cheirodontinae), Neotropical Ichthyology 16 (1), pp. 1-22 : 12-16

Odontostilbe microcephala Eigenmann, 1907

Figs. 10-12

Odontostilbe microcephalus Eigenmann, in Eigenmann, Ogle, 1907:10 (original description; type locality: rio Pilcomayo, Bolivia). - Fowler, 1940:61-62 (color description in alcohol; collected río Pilcomayo, tributary of the río Paraguay, Villa Montes, Department of Tarija).

Odontostilbe microcephala . - Eigenmann, 1910:429 (cited, habitat: Pilcomayo).- Eigenmann, 1915 (in part): 94-95 (type and paratype from río Pilcomayo, Bolivia); 94 (figure maxilla, premaxilla and mandible). - Ringuelet et al., 1967: 94 (key diagnosed), 95-96 (brief description).- Géry, 1972:70-71 (comparative material). - Géry, 1977:558 (key).- Casciotta et al., 1992 (comparative material). - Malabarba, 2003:218 (distribution: rio Pilcomayo basin, Bolivia). - Bührnheim, Malabarba, 2006:172 (comparative material). - Bührnheim, Malabarba, 2007:5 (comparative material). - Miquelarena et al., 2008: 57-58 (Key), 82-83 (description, distribution).- Mirande, Aguilera, 2009:181 (description, distribution) - Mirande, 2010:557 (comparative material).

Diagnosis. Odontostilbe microcephala is distinguished from all species of the genus by the subterminal mouth (vs. terminal mouth, except in O dierythrura , O. euspilurus , and O. avanhandava ); usually 10 gill rakers on upper branch and 14-15 on lower branch (vs. 11-12 on lower branch in O. dierythrura , 5-6 on upper branch and 9-10 on lower branch in O. euspilurus ). Odontostilbe microcephala differs from O. avanhandava in the horizontal orbit diameter (24.6-32.8 % HL, mean 28.7% vs. 31.0-39.7% HL, mean 35.5 %), number of scales rows between lateral line and dorsal-fin origin (6 vs. 5 in O. avanhandava ). Odontostilbe microcephala further differs from O. weitzmani by the presence of 12-13 lamellae in left and right sides of central median raphe of olfactory rosette with (vs. 20-21) and by the horizontal orbit diameter (24.6-32.8 % HL, mean 28.7% vs. 34.0-40.9% HL, mean 37.0 %). Additionally O. microcephala is distinguished from O. avanhandava and O. weitzmani by the absence of mesopterygoid teeth (vs. presence).

Description. Morphometric data in Tab. 3 . Body relatively elongate and compressed ( Fig. 10 ). Greatest body depth at or immediately anterior to dorsal-fin origin. Dorsal profile of head relatively convex from snout to vertical through posterior border of nares, straight or slightly convex from that point to rear of supraoccipital spine. Predorsal profile of body slightly convex from posterior end of supraoccipital to dorsal-fin origin located at vertical of midlength of standard length, dorsal-fin base straight to slightly convex. Dorsal profile of body between last dorsalfin ray and adipose-fin slightly convex, slightly concave from that point to caudal-fin origin. Caudal peduncle slightly longer than deep. Ventral profile of head slightly convex from mouth to pectoral-fin origin; convex from that point to ventral-fin origin, with an obtuse angle in the prepelvic region. Ventral profile straight from pelvic-fin origin to anal-fin origin. Anal-fin base straight to slightly concave. Caudal-peduncle ventral profile straight.

Head small. Mouth subterminal, upper jaw and snout extending anteriorly beyond lower jaw tip. Maxilla short, positioned obliquely at angle inferior of 45 degrees relative to body axis. Premaxillary dentition in single row with 5 teeth, slightly inclined to inside mouth, bearing 7 cusps, exceptionally 9; central cusp slightly longer and wider than lateral cusps. Maxilla with two teeth bearing 7 cusps, similar in shape to premaxillary teeth. Dentary with 8 or 9 teeth, anterior 7 teeth larger, bearing 7 cusps, with central cusp slightly longer and wider than lateral cusps, followed posteriorly by 1 or 2 smaller and conical teeth. Dentary teeth inclined anteriorly ( Fig. 11 ).

Dorsal-fin rays ii,9(23). Dorsal-fin origin posterior to vertical through pelvic-fin insertion. Profile of distal margin of dorsal fin slightly concave. First unbranched dorsal-fin ray about half-length of second ray, following branched rays gradually decreasing in size posteriorly. In mature males, second unbranched ray is elongated in filament; its length three times longer than first unbranched ray, reaching origin of adipose fin in some specimens ( Fig. 12b ). First unbranched ray of dorsal fin inserted in first pterygiophore and last 2 branched rays inserted in tenth pterygiophore. Proximal radial of first pterygiophore in contact with neural spine of 11th or 12th precaudal vertebrae. Dorsal fin with medial radial fused with distal radial from first to fifth pterygiophore and not fused from sixth to ninth pterygiophore. All proximal radials with lateral projections. Origin of adipose fin anterior to vertical through base of last anal-fin ray.

Unbranched anal-fin rays iv(1), or v(3); branched analfin rays 16(1), 17(6), 18(8), 19(7), with two rows of scales covering base of anterior 5 branched rays. Anal-fin origin posterior to vertical through base of last dorsal-fin ray. Profile of distal margin of anal fin concave. Anal fin with 20 pterygiophores, anterior 3 or 4 unbranched rays associated with first pterygiophore. First 4 to 7 pterygiophores with medial radial fused with distal, not fused from that point to last pterygiophore. Proximal radial of first pterygiophore in contact with hemal spine of first caudal vertebra.

Pectoral-fin rays i(23), 10(1), 11(11), 12(9), or 13(2). Longest pectoral-fin ray reaching anterior edge of pelvic bone, reaching origin of pelvic fin in mature males. Pelvic fin i,7(23) rays; fin insertion slightly anterior to vertical through origin of dorsal fin. In mature males, first unbranched ray prolonged in filament, usually surpassing origin of anal fin. Elongated thin hooks present on all branched pelvic-fin rays in mature males. Principal caudalfin rays 19(23). Procurrent caudal-fin rays: dorsal 9(5), 10(9), 11(5), or 12(3) and ventral 6(1), 7(1), 8(7), 9(10), or 10(3).

Cycloid scales; lateral line with 35(3), 36(4), or 37(8) pored scales. Predorsal scales arranged in regular series with 11(4), 12(5), or 13(1) scales; scale rows between lateral line and dorsal-fin origin 6(15); scale rows between lateral line and pelvic-fin origin 4(14), exceptionally 3(1); scale rows around caudal peduncle 14(23).

Supraneurals 5(11) or 6(3). Precaudal vertebrae 17(4) or 18(5); caudal vertebrae 17(4), 18(4), or 19(1); total vertebrae 34(1), 35(6), 36(1), or 37(1). Upper gill rakers 10, lower 14-15 (2 on hypobranchial). Upper gill rakers with 3 or 4 denticles on anterolateral border, and 2 or 3 denticles on posterolateral border. Lower gill rakers with 4 denticles on anterolateral border, and 1 or 2 denticles on posterolateral border. Denticulation mainly on basal portion of gill rakers. Olfactory rosette circular in shape and with 12 (2) or 13 (1) lamellae in left and right sides of central median raphe.

Color in alcohol. Overall body color pale yellow to light brown. Dorsal surface of head from snout to anterior limit of frontal pale yellow, black chromatophores noticeably more concentrated in region of parietal and supraoccipital bones. Region of third to fourth infraorbitals and opercular apparatus silver. Dorsum from posterior limit of supraoccipital to caudal peduncle with dark gray chromatophores more concentrated on scales border. Ventral region between pectoral and pelvic fins light yellow. Humeral region with slightly darkened triangular area due to muscular hiatus of pseudotympanum. Body with silver lateral band between pseudotympanum and caudal peduncle. Caudal peduncle with brown rhomboid spot, not reaching dorsal and ventral borders of caudal peduncle, covering basal portion of central rays of caudal fin.

Dorsal fin translucent, with few chromatophores scattered in anterior rays. Black chromatophores along first unbranched pectoral-fin ray. Pelvic fin hyaline. Anal fin hyaline with light gray chromatophores in basal portion of anterior rays. Adipose fin hyaline with light gray chromatophores basally. Caudal-fin ray hyaline with basal portion covered by caudal peduncle spot.

Coloration of paratype CAS 59791 badly preserved, characterized by long silver lateral band extending from region immediately posterior to pseudotympanum to near caudal-fin base.

Color in life. Caudal spot black, not reaching upper and lower margin of caudal peduncle and extending to base of caudal fin, followed by silvery pigments in each lobe. Medial part of dorsal and anal fins gold silvery. Lateral band dark green with golden line on upper margin, extending from pseudotympanum to caudal spot. Body silvery, with scattered dark chromatophores on scales and fins.

Sexual dimorphism. Sexually mature males with small hooks on anal and pelvic fins. Anal-fin rays ( Fig. 12c, d ) with one pair of retrorse bony hooks per ray segment symmetrically distributed, with robust base and curved or straight tips, present on last unbranched ray and anterior 6 to 8 branched rays, decreasing in number posteriorly. Bony hooks present on median portion of posterior branch, absent on anterior branch. Tip of bony hooks not reaching proximal border of segment of lepidotrichia where inserted. Pelvic-fin rays ( Fig. 12e,f ) with small hooks on all branched rays; hooks retrorse with robust base and curved tips inserted on median and distal partions of fin rays, some very small hooks on distal border of proximal portion. One bony hook, per segment on posterolateral border of anterior and posterior lepidotrichia in median portion and on posterolateral border of posterior lepidotrichia in distal portion. Tip of bony hooks not reaching proximal border of segment of lepidotrichia where inserted.

Geographic distribution. Odontostilbe microcephala is known from río Pilcomayo, río Paraguay basin, in Bolivia and Argentina and río Masicuri, headwaters of rio Mamoré, Amazon basin, in Bolivia ( Fig. 6 ). Carvalho, Albert, (2011) discuss the distribution of Odontostilbe microcephala between the Paraguay and Mamoré basins that is located mainly in the Chaco Plain and traversed by several major rivers (i.e., Parapetí, Grande, and Pilcomayo rivers), where typical taxa from the La Plata and upper Madeira basins occurs, due possibly to an exchange of water during the rainy season. O. microcephala is also distributed in the río Bermejo, río Juramento and río Sali-Dulce basins, in Northwestern Argentina ( Mirande, Aguilera, 2009) and arheic basin of río Horcones a tributary of the río Salado. The first two basins are parallel to the Pilcomayo and drains into the Paraguay and Paraná rivers. The Sali-Dulce basin is endorheic.

Ecological notes. Examination of gut contents of two specimens of Odontostilbe microcephala (USNM 305484) revealed the presence of insect larvae, composed of a high number of nymphs of Ephemeroptera and pupae of Diptera, followed by larvae of Trichoptera and allochthonous adults of Hymenoptera (Vespidae) .

Rondineli et al. (2011) described the species as insectivorous based on specimens collected from the rio Corumbataí (São Paulo, Brazil). The material examined by Rondineli and co-authors is possibly Odontostilbe avanhandava or O. weitzmani since O. microcephala does not occur in the upper rio Paraná. These authors, however, do not provide any descriptive information or photo of the fish he examined, not allowing confirmation of the identifications.

Conservation status. Odontostilbe microcephala is categorized as Least Concern (LC), according to the IUCN criteria for evaluation on threatening status, version 3.1 ( Reis, Lima, 2009), due to its wide distribution and the lack of any known major threats across its range. For this categorization, Reis, Lima (2009) not considered the individuals of the upper part of the Paraná basin (currently O. weitzmani and O. avanhandava ), due to this their current categorization of threat does not vary.

Remarks. Odontostilbe microcephala was briefly described by Eigenmann (in Eigenmann, Ogle, 1907) based on two specimens from the río Pilcomayo in Bolivia. Eigenmann (1915) redescribed the species with more detail based on the type specimens plus some specimens collected in the rio Tietê, São Paulo, Brazil (CAS 60508, FMNH 57871, FMNH 57872, FMNH 131317), and extended its distribution to the upper rio Paraná basin. Specimens examined by Eigenmann (1915), however, were analyzed herein and belong to the two new above described species, and no specimens of O. microcephala were found among the material available from the upper rio Paraná basin. Due to the redescription of Eigenmann (1915), the name of O. microcephala has been cited for the upper rio Paraná basin in different studies (e.g. Oyakawa, Menezes, 2011). This species is now restricted to the Andean slope of the río Paraguay and edorheic río Salí basin ( Argentina and Bolivia).

Material examined. Holotype CAS 59790, 1 , 46.0 mm SL, río Pilcomayo , Bolivia, tributary of río Paraguay, 21°11′32″S 63°47′32″W, 1900-1901 . Paratype CAS 59791, 1 , 45.0 mm SL, río Pilcomayo , Bolivia, tributary of río Paraguay, 20°43′30″S 64°12′55″W, 1900-1901 . Non-type specimens: MCP 38311, 4 , 38.9-43.6 mm SL, endorheic of río Uruena , drainage of Bajo Paraná, Rosario de la Frontera, Salta, Argentina, 25°00′00″S 64°30′00″W, 2 Mar 2001, G. Monasterio de Gonzo & M. Mosqueira . MCP 38310, 4 , 39.8-55.3 mm SL, arheic basin of río Horcones , Salta, Rosario de la Frontera, Argentina, 25°00´00″S, 64°30´00″W, 03 Mar 2001, G. Monasterio de Gonzo & M. Mosqueira . USNM 176033 , 1 , 48.9 mm SL, río Dulce , endorheic, afluent Mar Chiquita Lake Argentina, 29°38′47″S 62°52′19″W, 1 Aug 1933, T. Marini . UMSS 473 , 6 , 37.4 -43.0 mm SL, río Masicurí , farm Piraymiri , río Grande , río Mamoré , río Madera, río Amazonas, Bolivia, 18°50′36″S 63°45′32″W, 22 nov 2005, L. Cordova, M. Maldonado, M. Arraya , USNM 306349 , 2 , 41.7-47.4 mm SL, rio Bermejo , 4-5 km S, Pueblo Salado, 30 air KM NW Bermejo, border of Department of Tarija, río Paraguay, Argentina-Bolivia, 22°27′00″S 64°32′00″W, 5 Oct 1988, W. Starnes, L. Starnes, J. Sarmiento & R. Vasquez . USNM 305484 , 109 , 33.54-51.97 mm SL, río Pilcomayo , Department of Tarija , at Villamontes rr Bridge, río Paraguay, Bolivia, 21°16′48″S 63°28′12″W, 1 Oct 1988, W. Starnes, L. Starnes, J. Sarmiento & R. Vasquez . USNM 321173 , 49 , 29.7 -44.0 mm SL (2 c&s, 30.6- 44.0 mm SL), río Camatindi , 8 Km N Border Department of Tarija, 40 Km N Villamontes, Department Chuquisaca, riío Paraguay, Bolivia, 20°59′34″S 63°23′51″W, 2 Oct 1988 Cols W. Starnes, L. Starnes, J. Sarmiento & R. Vasquez . USNM 319279 , 200 , 21.96- 38.04 mm SL, rio Parapeti , at Rr bridge at San Antonio , 40 Air km E Camiri, Department Santa Cruz, Amazon and Paraguay, Bolivia, 20°01′12″S 63°12′00″W, 30 Sep 1988, W. Starnes, L. Starnes, J. Sarmiento & R. Vasquez . MHNG 2653.042 , 2 , 38.8-40.9 mm SL, Pazo Hondo , río Pilcomayo, Paraguay, 12 Aug 1994, C.Dlouhy .

Comparative material examined. Odontostilbe fugitiva : Perú: ANSP 178908, 2 c&s, 29.5-32.5 mm SL. Brazil: INPA 18506, 3 c&s, 24.5-32.4 mm SL. INPA 18512, 1 c&s, 34.9 mm SL. Ecuador: MZUSP 77844, 2 c&s, 36.9-40.0 mm SL. Odontostilbe pulchra : Trinidad and Tobago: INHS 40101, 3 c&s, 32.5-34.4 mm SL. INHS 40081, 1 c&s, 30.5 mm SL. Odontostilbe nareuda : Brazil: MZUSP 87759, 1 c&s, 27.5 mm SL. Bolivia: FMNH 106433, 1 c&s, 31.6 mm SL. Odontostilbe dierythrura : Bolivia: MCP 38624, 7 (2 c&s, 33.5-38.4 mm SL), 32.7-34.5 mm SL. Odontostilbe euspilurus : Perú: ANSP 143702, 2 c&s, 29.5-33.1 mm SL. Ecuador: MCP 38420, 13, 35.2-41.2 mm SL. Odontostilbe paraguayensis : Brazil: MCP 35618, 54, 28.5-33.9 mm SL. Paraguay: MCP 12031, 3, 30.92-33.90 mm SL. Odontostilbe pequira : Brazil: UFRGS 7022, 2 c& 36.7-38.7 mm SL, UFRGS 8641, 14, 32.0- 46.6 mm SL. UFRGS 5589, 5, 31.5-34.0 mm SL. UFRGS 13365, 22, 37.6- 33.9 mm SL. UFRGS 13006, 99, 26.6- 34.7 mm SL. Brazil. MZUSP 21067, 1 c&s, 31.0 mm SL .

Figures

Fig. 10. Odontostilbe microcephala: (a) holotype CAS 59790, male 32.7 mm SL, río Pilcomayo, Bolivia, tributary of río Paraguay (photo by The California Academy of Sciences - Ichthyology Section); (b) MCP 38310, female, 45.6 mm SL, arheic basin of río Horcones, Salta, Rosario de la Frontera, Argentina; (c) male, río Salí, in Tucumán, Argentina (not preserved, photo by Gaston Aguilera).

Fig. 11. Dentition of Odontostilbe microcephala, paratype USNM32473,left side, lateral view:(a) premaxilla,(b)maxilla, and (c) dentary. Scanning electron micrographs (SEM).

Fig. 12. Odontostilbe microcephala. Male (MCP 38311): (a) Subterminal mouth (b) Second unbranched of dorsal fin elongate. Male (USNM 321173): (c) and (d) hooks in anal-fin rays; (e) and (f) hooks in pelvic-fin rays.

Fig. 6. Southern South America showing the distribution of Odontostilbe avanhandava (green diamonds), O. microcephala (yellow squares) and O. weitzmani (red circle). Type localities represented by star of respective colors.